1. Field of the Invention
The present invention relates to hydraulic power systems with electrically operated control valves, and more particularly to electrical circuits that control the application of electricity to such valves.
2. Description of the Related Art
A wide variety of machines have movable members which are driven by a hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. For example, backhoes have a tractor on which is mounted a boom, arm and bucket assembly with each of those components being driven by one of more cylinder-piston arrangements. The flow of fluid to and from each hydraulic actuator is controlled by a hydraulic valve that traditionally was manually operated by the machine operator.
There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing, as the control valves do not have to be located near an operator station, but can be located adjacent the hydraulic actuator being driven by the fluid. This change in technology also facilitates computerized control of the machine functions.
Application of pressurized fluid from a pump to the hydraulic actuator is controlled by a set of electrohydraulic proportional pilot-operated valves. These valves employ a solenoid coil which generates a magnetic field that moves an armature in one direction to open a valve. The armature acts on a valve element which opens and closes a pilot passage that in turn causes a main valve poppet to move with respect to a primary valve seat located between the inlet and outlet of the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the solenoid coil, the electric current produces a variable magnetic field that moves the armature to open the pilot poppet to varying degrees, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or another component is spring loaded to close the valve when electric current is removed from the solenoid coil.
Magnetic hysteresis is the retention of magnetism induced in ferromagnetic materials and affects the operation of the valve as the applied electric current changes. For example, as the electric current decreases to close the valve the residual magnetism tends to keep the valve open slowing the response of the valve to the change in the electric current level. This phenomenon causes a difference between the flow of fluid through the valve that is desired and the actual flow.
Precise control of the electric current that is applied to the solenoid valve is essential for accurate control of the machine motion. However, the magnetic hysteresis adversely affects the precision of that control.